1. Field of the Invention
The present invention relates to an Y/C (luminance signal/chrominance signal) separator.
2. Description of the Prior Art
A conventional Y/C separating filter apparatus comprises a three-dimensional filter for obtaining a carrier chrominance signal by subtracting and averaging an NTSC composite color video signal formed of a low band luminance signal, a high band chrominance signal, one frame period-delayed and one frame period-advanced signals, a two-dimensional filter (comb filter) for obtaining a carrier chrominance signal by subtracting the composite color video signal and a signal having a different one horizontal cycle period, a one-dimensional filter (bandpass filter) having a frequency characteristic in which a carrier chrominance signal from the composite color video signal is passed, a selector circuit for selecting respective outputs of the three-dimensional, two-dimensional and one-dimensional filters, a motion detector circuit for detecting a motion of the composite color video signal, and a judging circuit for determining whether or not a line correlation of the composite color video signal is higher than the reference value. The selector circuit selects the output of the three-dimensional filter when the motion is not detected, and selects the outputs of the two-dimensional filter and the one-dimensional filter when the motion is detected. The selector circuit further selects the output of the two-dimensional filter when the line correlation of the composite color video signal is higher than the reference value, and the selector circuit selects the output of the one-dimensional filter when the line correlation of the composite color video signal is lower than the reference value to thereby obtain the carrier chrominance signal. The luminance signal is obtained by subtracting the carrier chrominance signal from the composite color video signal.
Based upon the above description of the prior art, let us consider the case where the contents of a composite color video signal of a still picture are the same up to the fields . . . , n-4, n-3, n-2, n-1 and that the contents thereof are changed from the next field n and that the same contents are continued in the fields n, n+1, n+2, n+3, . . . , as shown in FIG. 1 of the accompanying drawings.
With the above-mentioned arrangement, up to the fields n-2, n-1, n, n+1, one of the outputs which result from subtracting the composite color video signal from the one frame cycle period delayed and advanced signals becomes a signal which results from subtracting the still picture signals of the same content and the other becomes a content which results from subtracting the still picture signals of different contents. Accordingly, an average value of such signals becomes meaningless. Therefore, in that case, the selector circuit cannot select the output of the three-dimensional filter and must select the output of either the two-dimensional filter or the one-dimensional filter. There is then the risk that a cross-color or dot-interference occurs in a reproduced picture of such composite color video signal.